In a server and a personal computer, a heat spreader is attached to an electronic component such as a central processing unit (CPU) to dissipate heat generated in the electronic component to the outside.
When the thermal resistance between the heat spreader and the electronic component is high, heat in the electronic component cannot be transferred to the heat spreader quickly. For this reason, a heat dissipating sheet with excellent thermal conductivity is interposed between the electronic component and the heat spreader in some cases.
There are various types of heat dissipating sheets. An indium sheet is an example of the heat dissipating sheets. However, since the expensive indium is employed in the indium sheet, the cost of the heat dissipating sheets cannot be lowered. Moreover, indium has a thermal conductivity of 80 W/m·K, with which it is difficult to efficiently dissipate heat from an electronic component.
A thermal conductive polymer can be used as the heat dissipating sheet, but the thermal conductivity of a thermally conductive polymer is not sufficiently high.
To deal with these problems, a heat dissipating sheet using carbon nanotubes is under consideration as an alternative of the indium sheet and the thermally conductive polymer.
Carbon nanotubes have a thermal conductivity of about 1,500 W/m·K to 3,000 W/m·K, which is much higher than that of indium (50 W/m·K). Thus, carbon nanotubes are attractive for use in the heat dissipating sheet.
Although there are various proposed techniques for application of carbon nanotubes to a heat dissipating sheet, each of them have room for improvement.
For example, it is proposed to disperse carbon nanotubes into a resin to form the heat dissipating sheet. In this method, since the carbon nanotubes are oriented in various directions, it is difficult to transfer heat along the carbon nanotubes from the upper surface to the lower surface of the sheet.
Alternatively, it is also proposed to grow carbon nanotubes along the normal direction of the substrate, and then completely fill the spaces between the carbon nanotubes with resin. Since all the carbon nanotubes are oriented in substantially the same direction in this method, it is supposedly easy to transfer heat from the upper surface to the lower surface of the sheet along the carbon nanotubes.
In this method, however, resin is interposed between end portions of the carbon nanotubes and the electronic component, which increases the thermal resistance between the electronic component and the carbon nanotubes.
Note that techniques related to the present application are disclosed in the following literature:
Japanese Laid-open Patent Application No. 2005-150362;
Japanese Laid-open Patent Application No. 2006-147801;
Japanese Laid-open Patent Application No. 2006-303240;
Japanese Laid-open Patent Application No. 2010-118609;
Japanese Laid-open Patent Application No. 2010-267706;
Japanese Laid-open Patent Application No. 2012-199335;
Japanese Laid-open Patent Application No. 2010-199367;
Japanese Laid-open Patent Application No. 2013-115094; and
Japanese Laid-open Patent Application No. 2014-60252.